1. Field of the Invention
This invention relates to wired systems for controlling a plurality of remotely located loads from a central station employing digital messages sent over a common interconnecting bus.
2. Prior Art
In commercial and industrial facilities it is often desirable to control the application of power to a number of loads spaced about the facility from a central control point. The central control may be exercised manually, as by an operator who switches the loads on and off on the basis of measured operating criteria or a time schedule; or automatically by clock controlled circuitry or a central computer. In one form of system a computer monitors the electric power usage of the entire facility and automatically switches on and off various loads in accordance with an algorithm which regulates power consumption to minimize the charges for the electric power.
In these systems each of the remote loads has been directly wired to the central controller or relays connected to each load have been directly wired to the central controller.
In systems which exercise control over a large number of loads spaced about a facility, the wiring cost for such a system is very high and difficulties are encountered in maintaining the system because of the large number of wired runs.
A similar problem is encountered in remote terminal computer systems. Wiring runs to connect a central processing unit (CPU) with a large number of terminals located remotely about a facility can be very large and the problem is intensified when the wired connections must be of a type which will allow high rates of data transmission between the CPU and the terminals. In this environment the wiring problem has been solved by use of common bus structures employing a single cable connecting the CPU to all of the terminals, rather than separate wire runs from each of the terminals to the CPU. A signal transmitted on the bus by the CPU or by one of the terminals is received by all of the other elements connected to the system. Each terminal has a unique address and the destination of a message is determined by an address transmitted on the bus along with the message. Because of the high data transfer rates required in these computer systems, the common bus communication systems have employed very sophisticated and complex techniques including modulated carriers which necessitate the use of modems at each of the stations, and complex addressing and decoding schemes and the like. While these systems have enough versatility to allow their application to the relatively simplistic remote load control problem no economy would be achieved substituting such a sophisticated system for a wiring system wherein each load is connected to the controller by a separate line.